Cognitive radio allows different networks or users to dynamically share the frequency spectrum to achieve maximum utility without causing harmful interference to each other. To do so, a cognitive radio needs to frequently perform spectrum sensing, i.e. detecting the presence of primary users or other secondary users, such as, for example, other cognitive radios. Based on the detected spectrum information, a cognitive radio takes action to maximize its own cost function and at the same time protect the interests of primary users and other secondary users.
A cognitive radio does not have fixed spectra; therefore, it may need to sense a very large frequency range to find available spectrum. The sensed aggregate spectral bandwidth could be as large as several GHz. Furthermore, a cognitive radio may not have enough information of the band policy of primary users and other secondary users. Band policy may relate to rules on spectrum usage that users must obey. For example, the allocated center frequency and bandwidth for a television broadcaster are usually within a predefined set (called bands or channels). Therefore, when television signals are sensed in a large frequency range, sensing can be performed band-by-band, which is called multiband sensing.
However, in some other situations, the sensed primary and secondary users may not have such a policy or the policy may not be known. In other words, the sensed signal can be at any center frequency and occupy any bandwidth within the large frequency range. Lack of information of the center frequency and bandwidth of the sensed signal means that ‘band-by-band’ or ‘channel-by-channel’ sensing is not valid. Moreover, the center frequency and bandwidth of the sensed signal needs to be detected. This is called wideband sensing, which is more general than conventional multiband sensing wherein the center frequency and bandwidth of each band are known. Hence multiband sensing is just a special case of wideband sensing.